JP-2003-28586 A shows manufacturing method of a heat exchanger, which has a tube in which an inner fin is provided.
In this conventional method, a first plate and a second plate, which have a substantially U-shaped cross section, are combined with each other while holding an inner fin therein. After that, the inner fin and the first and second plate are bonded by brazing paste so that the tube having the inner fin therein is formed.
The brazing paste is applied on surfaces of the first plate and the second plate. Thereby, the inner fin is easily disposed inside of the tube.
However, in the above conventional manufacturing method of the heat exchanger, it is likely that a defective brazing may occur at the brazing portion of the inner fin. The inventor of the present invention realizes that such a defective brazing occurs for following reasons.
The brazing paste, generally, contains organic substances to make the brazing material in the form of a paste. When the first plate, the second plate, and the inner fin are heated to bond them together, the organic substances in the brazing paste on the first plate and the second plate evaporate before the brazing material melts.
The evaporating organic substances tend to remain the inside of the tube and hardly flow out from the tube since the first plate and the second plate are bonded. A part of the evaporating organic substances may form a layer on the inner surfaces of the first and second plate and the outer surface of the inner fin. This kind of layer deteriorates the wettability of the brazing material so that the defective brazing may be occurred at the brazing portion.